I believe a 401(k) with some pretax and some posttax money is now treated the same as a traditional IRA with a deduction claimed on some of the contributions but not others.

Here's how it works:

All of your IRAs are treated as if they were agglomerated into one big account. Any withdrawal from any IRA is partially untaxed according to the portion of the money you put in that was aftertax. You can't designate "I'm withdrawing only post-tax money".